Heretofore, an information recording medium (optical disc) capable of once recording information by a laser ray has been known. This information recording medium is also called a recordable CD (so-called CD-R) and is advantageous in that a small amount of CD can be swiftly provided at a reasonable cost as compared with the manufacturing of conventional CDs. Accordingly, demands for this recording medium are increasing with recent prevalence of personal computers.
A representative structure of the CD-R type information recording medium is such that a recording layer comprising an organic dye, a reflection layer comprising a metal, for example, gold, and a resin-made protective layer are laminated in this order on a transparent disc-like substrate. The recording of information on the optical disc is performed by irradiating a laser ray in the near infrared region (usually, a laser ray at a wavelength in the vicinity of 780 nm) to locally cause exothermic deformation of the recording layer. On the other hand, the reading (reproduction), of the information is usually performed by irradiating a laser ray having the same wavelength as that of the laser ray for recording and thereby detecting the difference in the reflectance between the exothermically deformed portion (recorded area) and the portion free of deformation (unrecorded area) of the recording layer.
Recently, an information recording medium having a higher recording density is demanded. In order to increase the recording density, reduction in the light size of the laser irradiated is effective and it is theoretically known that a laser ray having a short wavelength can be more reduced in the light size and is advantageous in the realization of a higher density. Therefore, studies are being made to develop an optical disc capable of recording and reproducing by a laser ray having a wavelength shorter than 780 nm which has been conventionally used. For example, an optical disc called a recordable digital video disc (so-called DVD-R) has been proposed. This optical disc is produced as follows: a disc is prepared by providing a recording layer comprising a dye on a transparent disc-like substrate having a diameter of 120 mm or 80 mm and having formed thereon a pregroove in a track pitch of 0.8 .mu.m narrower than 1.6 .mu.m of CD-R, and usually by further providing a reflection layer and a protective layer on the recording layer, and thereafter a couple of these discs or this disc and a disc-like protective substrate having almost the same dimension are laminated using an adhesive such that the recording layer is disposed in the inner side. On the DVD-R, the recording and reproduction are performed by irradiating a visible laser ray (a laser ray usually having a wavelength of from 600 to 700 nm), therefore, recording having a density higher than that of CD-R type optical disc can be attained.
The DVD-R type information recording medium can have a recorded information content a few times larger than that of conventional CD-R type information recording mediums, accordingly, the information recording medium is of course demanded to have a high recording sensitivity and in particular, since a large amount of information must be swiftly processed, is demanded to be reduced in the generation ratio of errors even at the high-speed recording.
Furthermore, since the recording layer comprising a dye is generally low in the aging stability against heat or light, a recording layer capable of maintaining stable performance against heat or light for a long period of time is demanded.
JP-A-63-209995 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") (corresponding to U.S. Pat. No. 4,968,593) and European Patent Publication No. 833,314A2 disclose a CD-R type information recording medium comprising a substrate having provided thereon a recording layer comprising an oxonol dye. By using this dye compound, the stable recording/reproduction properties can be maintained for a long period of time. As such a dye compound, an oxonol dye compound where ammonium in the form of a salt is introduced into the molecule is described.
The present inventors studied the performance of a DVD-R type information recording medium using an oxonol dye described in the above-described patent publication. As a result of the extensive investigation, it has been found that the DVD-R type information recording medium containing the oxonol dye in the recording layer exhibits relatively high recording properties but is not satisfied in the recording and reproduction properties because of low reflectance and low degree of modulation. Furthermore, reproduction failures were readily caused when the medium was exposed to light such as sunlight for a long period of time, thus, it was also found that the information recording medium is not satisfied in the light fastness.